Emergence: The Connected Lives of Ants, Brains, Cities, and Software, by Steven Johnson.
Emergence is one of those slippery ideas that is hard to define, other than to say “I know it when I see it”. Loosely it occurs when a system containing many interacting “atomic” components exhibits patterns at a higher level of abstraction, especially when these patterns are hard to explain in terms of atomic interactions. So the behaviour of the air in a balloon is fairly easy to describe as the average of the movement of all its individual molecules, but the behaviour of an ant colony is more than just the sum of the behaviours of the individual ants. We say the latter is “emergent” while the former is not.